Petroleum-based hydrocarbons, such as crude oil, can be separated into four fractions based on solubility in solvents: saturate, aromatic, resin, and asphaltene. Asphaltene is not considered to be defined by a single chemical structure, but is a complicated chemical compound. FIG. 1 depicts a model structure of asphaltene from Murray R. Gray, Consistency of Asphaltene Chemical Structures with Pyrolysis and Coking Behavior, Energy & Fuels 17, 1566-1569 (2003). Asphaltene is defined as a fraction which is not soluble in a n-alkane, particularly, n-heptane. The other fractions, including the resin fraction, which are soluble in n-alkane, are referred to as maltene.
The asphaltene fraction contains heteroatoms, which are compounds that include sulfur, nitrogen, oxygen or metals. Many heteroatom compounds are considered impurities and the goal of the refining process is to remove those impurities.
Metals are one of the impurities targeted for removal. Metals cause problems because they can be poisonous to the refining catalysts used to remove other impurities in the petroleum-based hydrocarbons. Metals also cause corrosion problems when combusted with hydrocarbons for power generation.
Another heteroatom impurity targeted for removal is sulfur. Sulfur in the asphaltene portion can be divided into two categories: aliphatic sulfides and aromatic thiophenes. The concentration of aliphatic sulfides and aromatic thiophenes in asphalthene depends on the type of petroleum from which the asphaltene is taken. Asphaltene derived from Arabian heavy crude oil has a total sulfur content of about 7.1 weight percent sulfur, including aliphatic sulfide above 3 weight percent. In other words, about half of the sulfur contained in asphalthene from Arabian heavy crude oil is aliphatic sulfides. In contrast, asphalthene from Maya crude oil has a total sulfur content of about 6.6 weight percent sulfur, where more than half of the total sulfur content is in the form of aliphatic sulfides.
Sulfur compounds contained in the heavy fraction can be converted to lighter sulfur compounds in the light fraction through dealkylation reactions or other reactions. The ability to convert the sulfur compounds to lighter compounds depends on the bond dissociation energy of the carbon-sulfur bonds. The bond dissociation energy of the carbon-sulfur bond depends on the type of the bond. For example, aliphatic sulfides have a lower bond dissociation energy than aromatic thiophenes. A lower bond dissociation energy means the aliphatic sulfides more easily generate radicals in thermal cracking than aromatic thiophenes. In fact, aliphatic sulfides are an important precursor for initiating radical reactions in thermal processing systems such as coker units. In addition, the breaking of aliphatic sulfide bonds generates hydrogen sulfide (H2S) as a main product. H2S is a known hydrogen transfer agent in radical mediated reaction networks.
Unlike the heavy crude oils, sulfur compounds in the light fraction, such as naphtha and diesel, are found as aromatic thiophenes. Aromatic thiophenes tend to be stable under thermal cracking conditions.
Sulfur compounds cause problems if released to the atmosphere and countries are imposing increasingly strict targets on the amount of sulfurs that can be released.
Current methods of addressing the presence of metals and sulfur include the use of additives and processing steps to remove the metals and the sulfurs from petroleum-based hydrocarbons. In one application, additives are injected to trap vanadium compounds in a combustor. While additives are effective to an extent, they cannot fully remove the metal compounds and therefore cannot completely prevent corrosion due to the presence of metals.
In conventional processing units, metal compounds and sulfur compounds can be removed from the crude oil itself or from its derivatives, such as refinery streams like residue streams. In a conventional hydroprocessing system, removal of impurity compounds is achieved by a hydroprocessing unit where hydrogen is supplied in the presence of a catalyst. Metal compounds decompose through reactions with hydrogen and are then deposited on the catalyst. Sulfur compounds decompose over the catalyst to produce H2S. The spent catalyst with the deposited metals is then regenerated in a regeneration unit. Alternately, following a period of operation the spent catalyst can be disposed of or destroyed. Although conventional hydroprocessing can remove substantial amounts of impurities from hydrocarbon streams, the process consumes huge amounts of hydrogen and catalyst. The short catalyst lifetime and huge hydrogen consumption contribute significantly to the costs associated with operating a hydroprocessing system. Large capital expenditures required to build a hydroprocessing unit coupled with the operating costs make it difficult for power generation plants to adopt such a complicated process as a pre-treatment unit of liquid fuel.
Another process that can be used to remove metals from petroleum-based hydrocarbons is a solvent extraction process. One such solvent extraction process is a solvent deasphalting (SDA) process. An SDA process can reject all or part of the asphalthenes present in a heavy residue to produce deasphalted oil (DAO). By rejecting the asphalthenes, the DAO has lower amount of metals than that of the feed heavy residue. The high removal of metals comes at the expense of liquid yield. For example, it is possible to reduce the metal content of an atmospheric residue from an oil crude from 129 ppm by weight (wt ppm) to 3 wt ppm in an SDA process, however the liquid yield of the demetallized stream is only around 75 percent by volume (vol %).
As noted above, catalytic hydrotreating can be used to remove sulfur from streams being used as a precursor to a coker unit. Although aliphatic sulfides are more active in catalytic hydrotreating than aromatic thiophenes, the complex of asphalthene prevents active sites on the hydrotreating catalyst from accessing the aliphatic sulfides, thus a very slow reaction ensues.
Porphyrin-type metal compounds can decompose in supercritical water. For example, vanadium porphyrin is known to decompose above 400° C. through a free radical reaction. The metal compounds produced as a result of the decomposition reactions in supercritical water reactions can include oxide and hydroxide forms. The metal hydroxide or metal oxide compounds can be removed by filtering elements installed downstream of the supercritical water reactor, such as between the supercritical water reactor and a separator. However, use of filters requires high energy usage to maintain the pressure differential necessary to maintain a high pressure drop across the filtering element. This configuration is also likely to end with a loss of valuable upgraded hydrocarbons that are absorbed onto the filtering elements.
Metals can be concentrated into certain parts of the petroleum products where the carbon to hydrogen ratio is higher than in other parts. For example, the coke or coke-like parts often contain highly concentrated metals. Specifically, vanadium can be concentrated into coke when heavy oil is treated with supercritical water under coking conditions, generally at high temperatures. Thus, although coke formation could be beneficial to remove metals from liquid phase oil products, there are problems caused by coke for example process lines are plugged by coke and liquid yield decreases with an increasing amount of coke.